A typical analogy explaining redshift involves a siren or a train whistle. So how do we know the same phenomenon works with light?
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Because both essentially travel in waves. And they can prove light redshift in other ways such as comparing something’s predicted spectral signature via its composition versus its observed one.
The doppler effect… We can see it happening with objects. Light behaves as a wave and we can observe it for near and far objects to prove it works the same way as a sound wave.
Well we know it happens because we can observe it. And use the effect quite a lot. Doppler radar is gonna be the big example. It’s used for weather stuff and radar speed guns.
The effect was first propose when Johann Doppler noticed shifts in light emitted by stars that corresponded to their motion. This was also a couple decades after the wave nature of light was discovered by observing interference patterns.
When energy propagates through a medium, it travels as a wave. This is true of light as well as sound. The fact that light behaves as a wave is well supported by scientific evidence (it also can behave as a particle in some situations, but in general it acts like a wave).
The Doppler Effect applies to all types of waves, so it applies to light.
If that isn’t enough, we have scientific evidence from observations of the universe that the doppler effect applies to light. In astronomy, we know that due to the expanding universe, stars or galaxies that are far away should be red shifted due to the Doppler Effect. When we evaluate all the stars and galaxies that we can see using telescopes, we find that the farther away they are, the redder they are, indicating a red shift is occuring.
Well, for one, we’ve observed it directly. Redshift isn’t theoretical, it’s a major tool for observing the Universe (both in the form of the Doppler effect for movement and in the form of cosmological redshift for distant objects).
But if you wanted to prove it without observation, you’d go back to the math of relativity. It turns out that for all observers to agree on certain facts, redshift has to occur. For example, suppose you are moving quickly and I am standing still, and a stationary source emits a light signal to both of us. In order for energy to be conserved (which it is, of course), you have to observe a redshifted signal relative to me.
Because the math works and it has been observed.
When light passes through a gas, some photons of specific wavelengths will be absorbed by the atoms. Each element has specific wavelengths that it absorbs due to electron configurations. The wavelengths that are absorbed will show up as a dark thin line in the spectrum of the observed light. The location and separation of these lines act as a fingerprint for the elements. This is how we can tell what elements make up stars.
When looking at distant stars, astronomers could see these spectral lines in the proper configuration, but they were all shifted towards the longer (redder) wavelengths. In theory these could be brand new elements that just looked like they were shifted, but Occam’s razor (and other evidence) makes this unlikely.
The red shift is why the James Webb Space Telescope is designed to operate in the infrared portion of the spectrum (it is NOT a visible light telescope). The JWST was designed to look at the most distant galaxies visible which are all in the infrared thanks to red shift.
We can measure it, an example you’ve likely heard the name of is “doppler radar”. Radio waves are just low frequency light, we can measure the shift in frequency based on how fast an object is moving towards or away from the radar, and we can measure the speed of that object with some other method to check that we’re measuring what we think we’re measuring.
Both can be seen as a sinusoidal wave (type it in google images). If the source of that wave (acoustic wave for sound, electromagnetic wave for light) moves toward a certain direction while emitting the wave, the peaks of the wave are going to be closer to each other (the distance between them, called wavelength, gets lower). Thus, to whoever receives that wave, it will be as if it was a wave of higher frequency than the frequency it was actually emitted at. (The higher the frequency of a sinusoidal wave you emit, the closer the peaks, which means a lower wavelength) It is called the Doppler effect. Hence why the pitch of an incoming siren sounds higher than how it would sound if the siren was stationary.
You can find short gifs illustrating that effect on the corresponding wikipedia page: [https://en.wikipedia.org/wiki/Doppler_effect](https://en.wikipedia.org/wiki/Doppler_effect)
Conversely, if the source is moving away from a given direction, the peaks of the wave are going to be more spaced from each other and thus, in the example of the siren, it will sound with a lower pitch than when it was approaching.
Similarly to the siren example, stars move while emitting light. Hence, to an observer towards whom the star is moving, the wavelength of the waves that observer receives, is going to be slightly decreased (shifted). Conversely, if the star is moving away from said observer, the wavelength of the waves the observer receives, is going to be slightly increased. I’m guessing that second case refers to redshift, given that among the range of wavelengths that correspond to light visible by the human eye (from 400 nanometers to 800 nanometers), red belong to the high wavelengths (around 800 nanometers).
Please note that, as much as what i’m saying regarding the siren example is correct, my analogy with the light example might be incorrect because that light example seem to be influenced not only by the simple Doppler effect but also by relativistic physics: [https://en.wikipedia.org/wiki/Redshift](https://en.wikipedia.org/wiki/Redshift) (which means, there are others phenomenons at play, that, from my impression, an analogy with the siren doppler effect might not suffice to explain)
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